This invention relates generally to medical implant devices, and more particularly to a biocompatible osteosynthesis plate that is deformable, yet dimensionally stable at its glass transition temperature, and relatively rigid at body temperature.
The repair of separated or dislocated bone fragments or segments following bone surgeries requires realignment of the separated or dislocated fragments or segments and subsequent secure fixation for promoting proper natural rejoinder of these bone fragments or segments. The presence of relative motion of the bone fragments or segments at a fracture or osteotomy location may result in irritation of the surrounding tissues, nonunion between the bone fragments, and an extension of the time of fracture healing. It is therefore desirable to accomplish as completely as possible an immobilization of the fracture or osteotomy site. This involves the relative fixation of affected bone segments relative to each other and in relation to the surrounding bone structure.
One example of an area in which such procedures are desirable is in the refixation of large area bone segments of the skull cap in neurosurgical and craniofacial operations on or through the vault of the human skull. In neurosurgical and craniofacial operations, one or more bone covers or segments of the skull are cut and removed. The operating technique employed in such operations includes the formation of several holes drilled through the cranial vault. A plurality of incisions, called connecting osteotomies, are then made between the drill holes by means of a saw which is provided with a guide device for avoiding dura mater injuries by the projection of the saw blade beyond the thickness of the skull. The number of holes drilled through the cranial vault, as well as the number of connecting osteotomies, is determined by the size, location and geometrical form of the bone cover to be lifted off. For example, if the bone cover to be removed is a triangular skull cap segment, three holes are preferably drilled at the corner points of the bone cover. The connecting osteotomies are made at the sides of a spherically curved triangle, resulting in a triangular skull cap segment which can be lifted off to carry out the further operation.
In the surgical treatment of craniofacial abnormalities, one or more bone segments of the skull cap may be removed and remodeled to achieve a desired cosmetic result before refixation in a displaced position relative to the surrounding bone. These operations serve to correct malformations of the skull cap which are present at birth, which operations are often performed during the infancy of the patient. These operations may include harvesting calvarian bone which is used to replace other bony tissues.
At the end of such procedures, the previously removed bone fragment or fragments are repositioned into their original locations, or in different desired locations, and are enhanced in a secured relation with other bone fragments removed during the surgical procedure and/or the surrounding bone portions. Known methods for providing fixation between adjacent bone portions have included the use of metallic plates of varying configurations, which are secured across osteotomies or fracture sites by metallic bone screws. These devices have been made of biocompatible metals and metal alloys, such as commercially pure titanium, stainless steel and cobalt chrome molybdenum. Other materials and devices, such as wires, intramedullary nails or externally fixed pins have also been used to reduce bone fracture mobility and to improve the relative position of adjacent segments. The aim of fixation of adjacent bone portions is to immobilize the fracture or osteotomy sites in order to promote localized bone growth in the natural repair of the separation.
The disadvantages associated with the use of metallic and metallic alloy devices relate to the undesirable cosmetic results associated with the protrusion of these devices above the bone surface, especially in locations directly beneath the skin, that is, without any intervening soft tissue for masking the implant devices from being noticed externally. As such, the only way to remove these implant devices involves revision surgery after the localized bone area has healed. In addition, metal and metallic alloy devices often should be removed from a pediatric patient so as to prevent growth restriction.
The use of medical implant devices made from bioresorbable materials has been described in literature and these devices have the advantage of being absorbed by the body over a period of time so as to allow for bone or fibrous material to become repaired at a fracture or osteotomy site by growing into the space created between adjacent bone portions. Many bioresorbable materials have been suggested for use in fixation of adjacent bone portions. It was believed that these materials had to be extremely strong to fixate the bone portions over a relatively long period of time. This typically meant that the osteosynthesis plate had to be relatively thick and be made out of a high molecular weight oriented material such as poly L-lactic acid in which the molecular weight would exceed 250,000. See Pihlajamaki, H., et al., "Absorbable Pins of Self-Reinforced Poly-L-Lactic Acid for Fixation of Fractures and Osteotomies," Journal of Bone and Joint Surgery, v. 74-B, n. 6, p. 853-857, November 1992. In addition, it was believed that copolymers of glycolide and lactide were not appropriate for use in osteosynthesis plates because of a rapid loss of mechanical strength. Grijpma, D. W., et al., "Poly (L-lactide) Crosslinked with Spiro-bis-dimethylene-carbonate," Polymer, v. 34, n.7, 1993 at 1496.
While others suggest the use of non-reinforced materials, the molecular weight of the material had to be increased to maintain strength. In this regard, one author suggested using a non-oriented material having an average molecular weight of 10.sup.6. See Bos, R. R. M., et al., "Late Complications Related to Bioresorbable Poly (L-Lactide) Plate - Osteosyntheses", Journal of Oral Maxillofacial Surgery, Supp. 3, 51(a) 1993 at 190. However, there were certain problems which were associated with these particular osteosynthesis plates. First, such osteosynthesis plates tended to have a relatively high degree of inflammatory response and therefore often had to be removed from the patient. See, Bostman, O., "Current Concepts Review--Absorbable Implants for the Fixation of Fractures," Journal of Bone and Joint Surgery, pp. 148-153, 1991. In addition, the osteosynthesis plates had to be made relatively thick so as to provide the requisite strength, which tended to make the osteosynthesis plates have an unwanted cosmetic appearance when implanted.
A need therefore exists for a bioresorbable fastening device for bone fixation, such as an osteosynthesis plate, that is thin enough and of a suitable material to be resorbed over a desired period of time, yet is of sufficient strength to maintain relative bone fixation over the time period needed for the natural repair of fractures or osteotomies between adjacent bone portions. A need also exists for a bioresorbable osteosynthesis plate which does not produce a substantial inflammatory response. In addition, a need also exists for such a bioresorbable device to be deformable, yet dimensionally stable at temperatures above the glass transition temperature of the material from which the device is made to facilitate shaping. A need further exists for such a bioresorbable device to allow for the formation of one or more additional fastener openings at one or more required precise locations during a surgical procedure.